<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
<script>
  /** 页面不会先渲染红色,而直接就是黄色(没有闪动)*/
  document.body.style = 'background:red';
  console.log(1);
  Promise.resolve().then(() => {
    console.log(3);
    document.body.style = 'background:yellow';
  });
  console.log(2);

  //123
</script>
</body>
</html>
